Дисертації: "Failure and Damage"

1

Shipsha, Andrey. "Failure of Sandwich Structures with Sub-Interface Damage." Doctoral thesis, Stockholm, 2001. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3184.

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2

DiPeri, Timothy P. "Neuromodulation Therapy Mitigates Heart Failure Induced Hippocampal Damage." Digital Commons @ East Tennessee State University, 2014. https://dc.etsu.edu/honors/208.

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Cardiovascular disease (CVD) is the leading cause of death in the United States. Nearly half of the people diagnosed with heart failure (HF) die within 5 years of diagnosis. Brain abnormalities secondary to CVD have been observed in many discrete regions, including the hippocampus. Nearly 25% of patients with CVD also have major depressive disorder (MDD), and hippocampal dysfunction is a characteristic of both diseases. In this study, the hippocampus and an area of the hippocampal formation, the dentate gyrus (DG), were studied in a canine model of HF. Using this canine HF model previously, we have determined that myocardial infarction with mitral valve regurgitation (MI/MR) + spinal cord stimulation (SCS) can preserve cardiac function. The goal of this study was to determine if the SCS can also protect the brain in a similar fashion. Both the entire hippocampus and the DG tissues were dissected from canine brains and analyzed. These findings provide strong evidence that, in addition to the cardioprotective effects observed previously, SCS following MI/MR induces neuroprotective effects in the brain.

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3

Alves, Marcilio. "Damage mechanics applied to structural impact." Thesis, University of Liverpool, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.484220.

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4

Wilkinson, Ann Elizabeth. "Skeletal muscle damage in patients with multiple organ failure." Thesis, University of Liverpool, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.283453.

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5

Chambers, Jeffrey Thomas. "Lengthscale effects in the damage and failure of composites." Thesis, Massachusetts Institute of Technology, 2014. http://hdl.handle.net/1721.1/90598.

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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Aeronautics and Astronautics, 2014.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 625-636).
The primary objective of this work is to investigate and identify lengthscale effects associated with damage in composite materials and their structures, and to determine how these lengthscales vary across levels of composites and can be used in assessing the overall response of composite structures. This is an advancement in a much larger pursuit towards developing a new methodology that utilizes composite failure and material data collected across all levels in order to predict the occurrence of damage and its effects at any operative level of composite structures. Documentation procedures are developed to capture qualitative and quantitative information on damage within experimental specimens, and computed microtomography provides additional information on the damage process. Specimens containing structural details are investigated postmortem to identify lengthscales associated with damage modes. Finite element models are developed in order to investigate the interaction of lengthscales associated with structural details with those associated with the basic damage modes. Based on these experimental and numerical results, lengthscales associated with five basic damage modes, as identified from previous studies, and the four structural details included in this investigation are identified and discussed, as are their interactions and importance. It is found that it is important to recognize two damage regimes, initiation and propagation, in characterizing lengthscales associated with damage modes. Identifying key lengthscales within each regime allows investigation of how the critical lengthscale(s) controlling the damage mode(s) change(s) across regimes. The concept of the "observable lengthscale" is identified as an important consideration when investigating lengthscales in experimental specimens and structures in that the observable lengthscale sets the ability to resolve damage and interactions of such. In a manner analogous to the "observable lengthscale," key lengthscales of basic damage modes and of structural details need to be used when choosing the scale of finite element models so that models have a resolution at least as fine as the key lengthscale of the mode under investigation. The results of the work show that the concept of lengthscales is a viable tool to characterize the overall response of composite structures, particularly involving damage initiation, damage propagation, and overall failure. The determination of how these lengthscales vary across levels in composites provides an important tool that can be used to assess this overall response of composite structures. Particular conclusions considering each damage mode are offered. In addition, a new damage type, called "transverse zigzag," is identified and studied, resulting in a finding that loads can "bypass" and "carry-through" regions of damage, depending on the geometry and laminate. Recommendations for further investigations are proposed based on the understanding of the role of lengthscales in the damage and failure of composites acquired from this work, and the needs identified to further this understanding.
by Jeffrey Thomas Chambers.
Ph. D.

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6

魏勇 and Yong Wei. "On fatigue failure prediction with damage mechanics: theory and application." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1993. http://hub.hku.hk/bib/B31233260.

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7

Halbert, Keith. "Estimation of probability of failure for damage-tolerant aerospace structures." Thesis, Temple University, 2014. http://pqdtopen.proquest.com/#viewpdf?dispub=3623167.

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The majority of aircraft structures are designed to be damage-tolerant such that safe operation can continue in the presence of minor damage. It is necessary to schedule inspections so that minor damage can be found and repaired. It is generally not possible to perform structural inspections prior to every flight. The scheduling is traditionally accomplished through a deterministic set of methods referred to as Damage Tolerance Analysis (DTA). DTA has proven to produce safe aircraft but does not provide estimates of the probability of failure of future flights or the probability of repair of future inspections. Without these estimates maintenance costs cannot be accurately predicted. Also, estimation of failure probabilities is now a regulatory requirement for some aircraft.

The set of methods concerned with the probabilistic formulation of this problem are collectively referred to as Probabilistic Damage Tolerance Analysis (PDTA). The goal of PDTA is to control the failure probability while holding maintenance costs to a reasonable level. This work focuses specifically on PDTA for fatigue cracking of metallic aircraft structures. The growth of a crack (or cracks) must be modeled using all available data and engineering knowledge. The length of a crack can be assessed only indirectly through evidence such as non-destructive inspection results, failures or lack of failures, and the observed severity of usage of the structure.

The current set of industry PDTA tools are lacking in several ways: they may in some cases yield poor estimates of failure probabilities, they cannot realistically represent the variety of possible failure and maintenance scenarios, and they do not allow for model updates which incorporate observed evidence. A PDTA modeling methodology must be flexible enough to estimate accurately the failure and repair probabilities under a variety of maintenance scenarios, and be capable of incorporating observed evidence as it becomes available.

This dissertation describes and develops new PDTA methodologies that directly address the deficiencies of the currently used tools. The new methods are implemented as a free, publicly licensed and open source R software package that can be downloaded from the Comprehensive R Archive Network. The tools consist of two main components. First, an explicit (and expensive) Monte Carlo approach is presented which simulates the life of an aircraft structural component flight-by-flight. This straightforward MC routine can be used to provide defensible estimates of the failure probabilities for future flights and repair probabilities for future inspections under a variety of failure and maintenance scenarios. This routine is intended to provide baseline estimates against which to compare the results of other, more efficient approaches.

Second, an original approach is described which models the fatigue process and future scheduled inspections as a hidden Markov model. This model is solved using a particle-based approximation and the sequential importance sampling algorithm, which provides an efficient solution to the PDTA problem. Sequential importance sampling is an extension of importance sampling to a Markov process, allowing for efficient Bayesian updating of model parameters. This model updating capability, the benefit of which is demonstrated, is lacking in other PDTA approaches. The results of this approach are shown to agree with the results of the explicit Monte Carlo routine for a number of PDTA problems.

Extensions to the typical PDTA problem, which cannot be solved using currently available tools, are presented and solved in this work. These extensions include incorporating observed evidence (such as non-destructive inspection results), more realistic treatment of possible future repairs, and the modeling of failure involving more than one crack (the so-called continuing damage problem).

The described hidden Markov model / sequential importance sampling approach to PDTA has the potential to improve aerospace structural safety and reduce maintenance costs by providing a more accurate assessment of the risk of failure and the likelihood of repairs throughout the life of an aircraft.

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8

Dannemann, Kathryn Ann. "Damage development and failure of fiber-reinforced ceramic matrix composites." Thesis, Massachusetts Institute of Technology, 1989. http://hdl.handle.net/1721.1/14197.

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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1989.
Vita.
Includes bibliographical references (leaves 111-120).
by Kathryn Ann Dannemann.
Ph.D.

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9

Chen, Boyang. "Numerical modelling of scale-dependent damage and failure of composites." Thesis, Imperial College London, 2013. http://hdl.handle.net/10044/1/24169.

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A computational study of the size effects of open-hole tension composite laminates is carried out. The thickness-dependence of translaminar fracture toughness is accounted for in the numerical model, which enables the sublaminate-scaling effect of strength to be accurately predicted by a deterministic model. Neglecting delamination in modelling is found to cause mesh-dependence and over-estimation in strength predictions. A smeared crack model with cohesive elements between plies can reliably predict the failure mode, but not the strength, for laminates failed by delamination. A floating node method is developed for explicitly modelling multiple discontinuities within an element. The degree-of-freedom vectors do not have associated initial coordinates; they are assigned to new material points when needed during analysis. The proposed method is well suited for modelling strong, weak and cohesive discontinuities, for the representation of complex crack networks, and for the accurate modelling of matrix crack/delamination interactions in composites.

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10

Sedman, Andrew James. "Mechanical failure of bone and antler : the accumulation of damage." Thesis, University of York, 1993. http://etheses.whiterose.ac.uk/14047/.

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11

Chen, Fuh-Sheng. "Damage and failure analysis of continuous fiber-reinforced polymer composites." Case Western Reserve University School of Graduate Studies / OhioLINK, 1992. http://rave.ohiolink.edu/etdc/view?acc_num=case1056554068.

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12

Berthier, Estelle. "Quasi-brittle failure of heterogeneous materials : damage statistics and localization." Thesis, Paris 6, 2015. http://www.theses.fr/2015PA066588/document.

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Nous proposons une nouvelle approche inspirée des modèles d'endommagement non-locaux pour décrire la ruine des matériaux quasi-fragiles désordonnés. Les hétérogénéités matériaux sont introduites à une échelle continue mésoscopique via des variations spatiales de la résistance à l'endommagement alors que le mécanisme de redistribution des contraintes est décrit à travers une fonction d'interaction que l'on peut faire varier. L'évolution spatio-temporelle de l'endommagement est déterminée à partir du principe de conservation d'énergie et caractérisée via une étude statistique des précurseurs à la rupture. Cette approche nous permet de prédire la valeur des seuils de localisation et de rupture en fonction de la nature des redistributions. A l'approche de la rupture, nous mettons également en évidence une augmentation en loi de puissance du taux d'énergie dissipée ainsi qu'une longueur de corrélation, supportant l'interprétation de la rupture quasi-fragile comme un phénomène critique. En effet, nous démontrons que notre model d'endommagement s'apparente à la loi d'évolution d'une interface élastique évoluant dans un milieu désordonné. Cette analogie nous permet d'identifier les paramètres d'ordre et de contrôle de cette transition critique et d'expliquer les invariances d'échelle des fluctuations dans la limite champ moyen. Enfin, nous appliquons ces concepts théoriques à travers l'étude expérimentale de la compression d'un empilement bidimensionnel de cylindres élastiques. Notre approche permet de décrire de façon quantitative la réponse mécanique non-linéaire du matériau, et en particulier la statistique des précurseurs ainsi que la localisation des déformations
We propose a novel approach inspired from non-local damage continuum mechanics to describe damage evolution and quasi-brittle failure of disordered solids. Heterogeneities are introduced at a mesoscopic continuous scale through spatial variations of the material resistance to damage. The central role played by the load redistribution during damage growth is analyzed by varying the interaction function used in the non-local model formulation. The spatio-temporal evolution of the damage field is obtained from energy conservation arguments, so that the formulation is thermodynamically consistent. We analytically determine the onsets of localization and failure that appear controlled by the redistribution function. Damage spreading is characterized through a complete statistical analysis of the spatio-temporal organization of the precursors to failure. The power law increase of the rate of energy dissipated by damage and an extracted correlation length close to failure supports the interpretation of quasi-brittle failure as a critical phenomena. Indeed, we establish a connection between our damage model and the evolution law of an elastic interface driven in a disordered medium. It allows to identify the order and control parameters of the critical transition, and capture the scale-free statistical properties of the precursors within the mean field limit. Finally, we experimentally investigate the coaction of localized dissipative events and elastic redistributions in disordered media via compression experiments of two-dimensional arrays of hollow soft cylinders. Our experimental observations show a quantitative agreement with the predictions derived following our approach

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13

Wei, Yong. "On fatigue failure prediction with damage mechanics : theory and application /." [Hong Kong : University of Hong Kong], 1993. http://sunzi.lib.hku.hk/hkuto/record.jsp?B13420410.

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14

Cao, Caihua. "Damage and Failure Analysis of Co-Cured Fiber-Reinforced Composite Joints." Diss., Georgia Institute of Technology, 2003. http://hdl.handle.net/1853/5298.

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Joints represent a design challenge, especially for composite structures. Among the available joining methods, co-curing is an efficient way to integrate parts for some applications. Coates and Armanios have proposed a Single Nested Overlap (SNO) co-cured joint configuration, obtained from a single lap joint through the overlap/interleafing of the adjoining top/bottom adherend plies, respectively. Through a comparative investigation, they have demonstrated joint strength and fatigue life improvements over the single lap joint counterparts for unidirectional and quasi-isotropic adherend lay-ups. This research extends the comparative investigation of Coates and Armanios by focusing upon characterizing and differentiating the damage initiation and progression mechanisms under quasi-static loading. Six specimen configurations are manufactured and tested. It is confirmed that single nested overlap joints show 29.2% and 27.4% average improvement in strength over single lap counterparts for zero-degree unidirectional and quasi-isotropic lay-ups, respectively. Several nondestructive evaluation techniques are used to observe and analyze damage initiation, damage progression and failure modes of the studied specimens and to monitor their mechanical response. Using X-ray Radiography and Optical Microscopy techniques during quasi-static loading, a physical characterization of damage and failure mechanisms is obtained. The acoustic emission data acquired during monotonic loading could reveal the overall picture of AE activities produced by the damage initiation, development and accumulation mechanisms within the specimen via parametric analysis. Further AE analysis by a selected supervised clustering method is carried out and shown successful in differentiating and clustering the AE data. Correlation with physical observations from other techniques suggests that the resulting clusters may be associated to specific damage modes and failure mechanisms.

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15

Greve, Lars. "Damage and failure modelling of carbon/epoxy Non Crimp Fabric composites." Thesis, Cranfield University, 2005. http://dspace.lib.cranfield.ac.uk/handle/1826/10710.

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Advanced material models for the intra- and inter-laminar damage and failure prediction of biaxial carbon/epoxy Non Crimp Fabric (NCF) composites are presented, which enable application to large scale practical composite structures for the automotive industry. The model established for intra-laminar failure combines the elasto-plastic continuum damage constitutive model first proposed by Ladevèze, with the matrix failure model of Puck, and improves the shear damage representation by using an exponential damage evolution law. An extensive test program has been conducted using biaxial NCF composites with differing degrees of fabric pre-shear. The presheared biaxial NCF are considered to take into account potential changes of the damage and failure properties occurring due to the draping process in real structures. The database established is used to identify parameters for the proposed Ladevèze-Puck matrix damage and failure model, allowing prediction of the main matrix failure modes of unsheared and presheared biaxial NCF composites. Furthermore, the failure prediction within zones of stress concentrations is addressed utilising flat NCF laminates with different holes and lay-ups. For numerical elastic failure prediction of these tests a simple fibre criterion is presented which is related to a characteristic element size. The Puck failure model is adopted to represent the inter-laminar (delamination) crack initiation strength of the investigated NCF. This stress based model for delamination crack initiation operates in conjunction with existing methods for the simulation of delamination crack propagation, which are related to the critical strain energy release rate. All material models that have been combined and further developed during the course of this work have been implemented in a research version of a commercial crashworthiness Finite Element code. The intra-laminar failure model is validated against a test series of transversely loaded circular- composite discs having differing degrees of fabric pre-shear. The final validation example for intra-laminar failure has considered a complex composite door structure subjected to lateral punch intrusion. The enhanced delamination failure model is validated using a composite beam structure, exhibiting delamination failure under local transverse tension loading. For all validation tests a good agreement between simulations and experiments has been demonstrated. It is believed by the author that the new contributions presented in this thesis significantly raise the level of numerical prediction of deformation, damage, and failure for Continuous Carbon Fibre Reinforced Composites. Special emphasis is placed on the fact that all material model parameters have been obtained from simple specimen tests, and have been used without adjustment for the simulation of subsequent Validation tests utilising Composite structures of high complexity. This continuous approach is in contrast to the majority of similar investigations found in the literature, in which the adjustment of material model parameters appeared to be necessary in order to achieve a good agreement between simulations and experiments of complex structures.

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16

Fouinneteau, M. R. C. "Damage and failure modelling of carbon and glass 2D braided composites." Thesis, Cranfield University, 2006. http://dspace.lib.cranfield.ac.uk/handle/1826/1555.

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Composite materials have been increasingly used in the past two decades since they offer significant potential weight reduction, part design flexibility and improved specific mechanical performance compared to traditional metals. For specific applications, braid reinforced composites offer better near net shape part and manufacturing flexibility than conventional unidirectional laminates, albeit at the expense of slightly lower in-plane stiffness and strength. Furthermore, for impact and crash applications, which is the emphasis of this thesis, their tow waviness and interlocking can offer excellent damage tolerance and energy absorption. In this work, heavy tow (24k) biaxial carbon and glass braided preforms were used to manufacture coupons and beam structures to undertake an extensive testing campaign to characterise different damage and failure mechanisms occurring in braided composites. Due to large shear deformation and surface degradation, non conventional measurement techniques based on marker tracking and Digital Image Correlation were successfully used to measure strains in the damaging material. The modelling of braided composites was conducted using the meso-scale damage approach first proposed by P. Ladevèze for unidirectional composites. The calibration of an equivalent braid unidirectional ply was achieved using the experimental results obtained for different braided coupons. Furthermore, failure mechanisms observed experimentally, such as tow stretching and fibre re-orientation occurring during loading history, were integrated into the model. A new unidirectional ply formulation was subsequently implemented into the explicit finite element code PAM-CRASHTM. Validation of the new model using single element, coupons and beams were conducted that provided a satisfying correlation between experimental tests and numerical predictions.

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17

Whitehouse, Anne Frances. "Damage and failure of discontinuously reinforced aluminium composites during tensile deformation." Thesis, University of Cambridge, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.319543.

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18

Manger, Christopher I. C. "Failure of notched woven GFRP composites : damage analysis and strength modelling." Thesis, University of Surrey, 1999. http://epubs.surrey.ac.uk/738/.

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19

Greve, Lars. "Damage and failure modelling of carbon/epoxy non-crimp fabtic composites." Thesis, Cranfield University, 2005. http://dspace.lib.cranfield.ac.uk/handle/1826/10710.

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20

Ribeiro, Marcelo Leite. "Damage and progressive failure analysis for aeronautic composite structures with curvature." Universidade de São Paulo, 2013. http://www.teses.usp.br/teses/disponiveis/18/18148/tde-18112013-140301/.

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Recent improvements in manufacturing processes and materials properties associated with excellent mechanical characteristics and low weight have became composite materials very attractive for application on civil aircraft structures. However, even new designs are still very conservative, because the composite structure failure phenomena are very complex. Several failure criteria and theories have been developed to describe the damage process and how it evolves, but the solution of the problem is still open. Moreover, modern manufacturing processes, e.g. filament winding, have been used to produce a wide variety of structural shapes. Therefore, this work presents the development of a damage model and its application to simulate the progressive failure of flat composite laminates as well as for composite cylinders made by filament winding process. The proposed damage model has been implemented as a UMAT (User Material Subroutine) and VUMAT (User Material Subroutine for explicit simulations), which were linked to ABAQUSTM Finite Element (FE) commercial package. Progressive failure analyses have been carried out using FE Method in order to simulate the failure of filament wound composite structures under different quasi-static and impact loading conditions. In addition, experiments have been performed not only to identify parameters related to the material model but also to evaluate both the potentialities and the limitations of the proposed model.
As recentes melhorias nos processos de fabricação e nas propriedades dos materiais associadas a excelentes características mecânicas e baixo peso tornam os materiais compósitos muito atrativos para aplicação em estruturas aeronáuticas. No entanto, mesmo novos projetos, ainda são muito conservadores, pois os fenômenos de falha dos compósitos são muito complexos. Vários critérios e teorias de falha têm sido desenvolvidos para descrever o processo de dano e sua evolução, mas a solução do problema ainda está em aberto. Além disso, técnicas modernas de fabricação, como o enrolamento filamentar (filament winding) vêm sendo utilizadas para produzir uma ampla variedade de formas estruturais. Assim, este trabalho apresenta o desenvolvimento de um modelo de dano e a sua aplicação para simular a falha progressiva de estruturas planas e cilíndricas fabricadas em material compósito através do processo de filament winding. O modelo de dano proposto foi implementado como sub-rotinas em linguagem FORTRAN (UMAT-User Material Subroutine e, VUMAT-User Material Subroutine para simulações explícitas), que foram compiladas junto ao programa comercial de Elementos Finitos ABAQUSTM. Várias análises numéricas foram realizadas via elementos finitos, a fim de prever a falha dessas estruturas de material compósito sob diferentes condições de carregamentos quase-estáticos e de impacto. Além disso, vários ensaios experimentais foram realizados, a fim de identificar os parâmetros relacionados com o modelo de material, bem como avaliar as potencialidades e as limitações do modelo proposto.

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21

Wang, Xiaofeng. "Computational technology for damage and failure analysis of quasi-brittle materials." Thesis, University of Manchester, 2015. https://www.research.manchester.ac.uk/portal/en/theses/computational-technology-for-damage-and-failure-analysis-of-quasibrittle-materials(a7c91eb6-5058-4e73-95de-b2f3efd645d2).html.

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The thesis presents the development and validation of novel computational technology for modelling and analysis of damage and failure in quasi-brittle materials. The technology is demonstrated mostly on concrete, which is the most widely used quasi-brittle material exhibiting non-linear behaviour. Original algorithms and procedures for generating two-dimensional (2D) and three-dimensional (3D) heterogeneous material samples are developed, in which the mesoscale features of concrete, such as shape, size, volume fraction and spatial distribution of inclusions and pores/voids are randomised. Firstly, zero-thickness cohesive interface elements with softening traction-separation relations are pre-inserted within solid element meshes to simulate complex crack initiation and propagation. Monte Carlo simulations (MCS) of 2D and 3D uniaxial tension tests are carried out to investigate the effects of key mesoscale features on the fracture patterns and load-carrying capacities. Size effect in 2D concrete is then investigated by finite element analyses of meso-structural models of specimens with increasing sizes. Secondly, a 3D meso-structural damage-plasticity model for damage and failure analysis of concrete is developed and applied in tension and compression. A new scheme for identifying interfacial transition zones (ITZs) in concrete is presented, whereby ITZs are modelled by very thin layers of solid finite elements with damage-plasticity constitutive relations. Finally, a new coupled method named non-matching scaled boundary finite element-finite element coupled method is proposed to simulate crack propagation problems based on the linear elastic fracture mechanics. It combines the advantage of the scaled boundary finite element method in modelling crack propagation and also preserves the flexibility of the finite element method in re-meshing. The efficiency and effectiveness of the developed computational technology is demonstrated by simulations of crack initiation and propagation problems.

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22

Mascarenhas, Wesley Novaes. "A Damage model for the ductile failure analysis of plastic components." Florianópolis, SC, 2011. http://repositorio.ufsc.br/xmlui/handle/123456789/95895.

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Tese (doutorado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Engenharia Mecânica, Florianópolis, 2011
Made available in DSpace on 2012-10-26T06:55:21Z (GMT). No. of bitstreams: 1 292560.pdf: 8869260 bytes, checksum: 119e3987b4e602d2a0ca1e47c42b6231 (MD5)
O objetivo deste trabalho é propor um modelo matemático e um sistema computacional, baseado no método dos elementos finitos, capaz de auxiliar a atividade de análise estrutural de componentes de plástico submetidos à falha dúctil. Basicamente, este sistema computacional será composto por um modelo elasto-viscoplástico acoplado ao dano, a fim de ser possível descrever a evolução das deformações inelásticas e os processos degradativos que conduzirão à falha e, assim, auxiliar no dimensionamento de componentes. O referido modelo foi desenvolvido sob as teorias propostas por Lemaitre [1], Fremond e Nedjar [2] e sob as abordagens da termodinâmica dos meios contínuos e do método do estado local, onde foram introduzidos os conceitos de potenciais termodinâmicos e de variáveis de estado. Ao se utilizar estas abordagens, assegura-se que os modelos produzirão resultados termodinamicamente consistentes. A fim de certificar o modelo proposto e validar o sistema numérico, realizou-se um ensaio uniaxial, em que o corpo de provas foi deformado até a sua fratura. Os dados obtidos do referido ensaio e de outros ensaios complementares foram utilizados na identificação das propriedades e constantes materiais, que definem o modelo proposto. Os procedimentos experimentais foram realizados no Departamento de Fibras e Tecnologia de Polímeros do Instituto Real de Tecnologia (KTH), em Estocolmo, Suécia.
The objective of this work is to propose a mathematical model and a numerical scheme, based on the finite element method, to be used to analyze mechanical components manufactured with plastic material, subjected to monotonic loading conditions that undergo a ductile failure. Basically, this numerical scheme will be composed by an elasto-viscoplastic model coupled with a non-local damage theory, in order to describe the evolution of the inelastic strains and the damaging processes of the material, which will consequently lead to the failure of the component. The cited model has been developed by means of the theories proposed by Lemaitre [1], Fremond and Nedjar [2], among others, and makes use of the method of local state variables and is derived within the scope of the consistent thermodynamics of the continuum medium. The main advantage of using these approaches is the confidence they give that the models can not produce thermodynamically unreasonable results. In order to attest the proposed model and to validate the numerical scheme, one considers an experimental uniaxial test, in which the specimen is deformed up to its ductile fracture. The data obtained from the given experimental uniaxial test and from other complementary experimental tests is then employed for the identification of the material properties and parameters, which define the proposed damage model. The experimental procedures have been performed at the Fibre and Polymer Technology Department of the Royal Institute of Technology (KTH), in Stockholm, Sweden.

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23

Anwar, Khurshid. "Role of apoptosis (programmed cell death) in acute liver failure." Thesis, University of Surrey, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.370058.

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24

Björklund, Oscar. "Modelling of failure." Thesis, Linköping University, Department of Management and Engineering, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-11466.

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This report is a review of some failure models today used for determine failure in thin sheets of high strength steels. Focus has been given on phenomenlogical models and only some simple simulations have been carried out. The phenomenlogical models that have been summarized here are of four different categories, namely stress based, strain based, combined stress and strain based and damaged models. However, the simulations have only been preformed for some of the models.

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25

Morgan, Stephanie. "How do chemotherapeutic agents damage the ovary?" Thesis, University of Edinburgh, 2014. http://hdl.handle.net/1842/9543.

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Chemotherapy treatment in premenopausal women has been linked to premature ovarian failure (POF), and hence infertility, through ovarian follicle loss. The exact mechanisms that lie behind this loss are unclear and so the action of two commonly used chemotherapeutic agents were compared here. Cisplatin is a DNA cross-linking agent commonly used in the treatment of ovarian, lung and bladder cancers, while the anthracycline doxorubicin is commonly used to treat leukaemia and breast cancer. Neonatal mouse ovaries were cultured in vitro and exposed to cisplatin or doxorubicin in order to determine their effects on primordial and early growing follicles. Both drugs caused a dose dependant follicle loss but targeted different cell types. Cisplatin caused a significant increase in follicles with unhealthy oocytes; furthermore primary stage follicles were the follicle class most affected (up to 98% classified as unhealthy compared with 13% in control, p<0.001). In contrast, doxorubicin caused a significant increase in follicles with unhealthy granulosa cells and affected all follicle stages present. When the mechanism of cell death was further investigated, apoptosis was the main pathway through which these drugs cause ovarian cell death. Doxorubicin in particular caused a significant increase in apoptosis of ovarian somatic cells including the granulosa cells and stroma. Imatinib mesylate, a tyrosine kinase inhibitor which is also used as a chemotherapeutic agent, has been implicated as a potential therapy to block the ovotoxic effects of cisplatin. Results here confirm this finding (29% of follicles classified as unhealthy in the cisplatin only group compared to 8% in the cisplatin and imatinib co-treatment group, p<0.001) and found further, that imatinib was unable to protect against doxorubicin-induced damage (28% of follicles classified as unhealthy in the doxorubicin treated group compared to 19% in the doxorubicin and imatinib cotreatment group). Imatinib treatment alone in newborn ovaries caused a significant increase in the number of follicles present at the end of culture compared to control (402±43 in the imatinib group compared to 188±34 in control, p<0.001), which is likely due to an effect on follicle formation. In conclusion, the work presented in this thesis demonstrates drug specific actions of cisplatin and doxorubicin on the mouse ovary. This suggests that any therapy designed to confer ovarian protection in the future may have to be tailored to be drug specific.

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26

Shaw, John Henry. "Effects of Fiber Architecture on Damage and Failure in C/SiC Composites." Thesis, University of California, Santa Barbara, 2015. http://pqdtopen.proquest.com/#viewpdf?dispub=3682976.

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Carbon-fiber/SiC-matrix composites are under development for applications in hypersonic vehicles due to their exceptional capabilities at high temperatures. As a subset of these materials, textile-based composites are of particular interest because they offer the possibility of accommodating complex geometries and features in engineering components. Among the numerous obstacles hindering the widespread adoption of these composites, two are addressed in the present work: (i) the incomplete understanding of the influence of textile architecture on thermoelastic properties, damage initiation and failure, and (ii) the lack of robust computational tools for predicting their thermomechanical performance at the appropriate length scales. Accordingly, an experimental study is performed of the thermal and mechanical properties of several prototypical textile C/SiC composites with various fiber architectures. In turn, the experimental results are used to guide the development of computational tools for predicting composite response that explicitly account for fiber architecture.

Textile architecture is found to influence composite response at four length-scales: the panel, the coupon, the tow, and the sub-tow. At the panel scale, distortions to the architecture introduced during weaving or handling of the fabric influence the packing density and the relative rotation of tows. Even when large distortions are intentionally introduced their influence on mechanical response is minimal. At the coupon scale the tow architecture has the largest effects on composite mechanical response. Young's modulus, ultimate tensile strength, and strain to failure are all influenced. Changes in each of these are a function of tow shape, tow anisotropy, and the degree of constraint provided by the matrix. At the tow scale, architecture effects give rise to heterogeneity in measured surface strains under both tensile and thermal loading. Methods for the calibration of tow-scale elastic and thermoelastic properties were developed to enable simulation of these effects with a geometrically-accurate virtual model. Virtual tensile and thermal tests using this model have indicated that interaction between tows has an important influence on local strains. At the sub-tow scale, architecture effects influence the location of matrix cracking. Simulations of the cooling cycle following matrix processing predict that matrix cracks should develop in the matrix above underlying tows due to thermal expansion mismatch between the tows and the matrix. This is consistent with experimental observations. Two methods are presented to extend the virtual tests to explicitly simulate the onset and evolution of these cracks.

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27

Amaya, Peter. "Progressive Damage and Failure Model for Composite Laminates under Multiaxial Loading Conditions." The Ohio State University, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=osu1338381439.

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28

Kholkin, A. "Numerical simulation of damage and failure of laminated 3-point bending specimens." Master's thesis, Universidade de Aveiro, 2012. http://hdl.handle.net/10773/10013.

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Mestrado em Engenharia Mecânica
Polímeros reforçados por fibras (PRF) tem sido utilizados durante muito tempo para aplicações estruturais, particularmente com laminados. No projeto de laminados é importante caracterizar o seu comportamento mecânico. O comportamento mecânico é caracterizado por uma rigidez inicial e pela fractura. A estratégia de modelação utilizada nesta dissertação permite não só prever o “first-ply-failure”, como também modelar a sequência de eventos a seguir, como a delaminação e a fractura final da estrutura. Para isso é utilizado um modelo para a interface e para as camadas, permitindo a interação entre eles. A rigidez inicial também é avaliada com diferentes ratios comprimento para espessura, permitindo também diferentes contribuições do cisalhamento transversal e de flexão. Nesta dissertação com o método dos elementos finitos (FEM) é investigado o comportamento estrutural de uma viga compósita sobe ensaios de flexão em três pontos. Para este fim o programa comercial Abaqus é utilizado, permitindo simulações numéricas com elementos continuum casca e elementos convencionais casca. Também são realizadas simulações numéricas de modo a preparar a resultados para uma futura experiência, para o qual o tamanho do provete é escolhido com recomendações do EN ISO 14125
Fiber-reinforced polymers (FRP) have been used for a long time for structural applications, particularly with laminates. It is important to characterize the mechanical behavior of laminates for their design. This behavior is characterized by an initial stiffness and by failure. The modeling strategy used in this thesis allows to predict not only first-ply-failure (FPF), but also to model the sequence of events afterwards, such as delamination and the final failure of the structure. This is achieved by using a damage model for the ply and for the interface, allowing the interaction between them. The initial stiffness is also evaluated with different length to thickness ratios, allowing different contributions from transverse shear and bending. In this thesis, with the finite element method (FEM) the structural behavior of a laminate composite beam under a three point bending configuration is investigated. For this purpose the commercial FEM package Abaqus is used, allowing numerical simulations with continuum shell and conventional shell elements. Also simulations are conducted, in order to prepare a future experiment for which the specimen size was chosen with recommendations from EN ISO 14125.

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29

LAMON, FEDERICO. "CHARACTERISATION AND MODELLING OF THE FATIGUE DAMAGE AND FAILURE IN WOVEN COMPOSITES." Doctoral thesis, Università degli studi di Padova, 2022. https://hdl.handle.net/11577/3467167.

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The PhD thesis aims to deepen the fatigue behaviour of textile composites, with particular focus on woven fabrics. The high specific stiffness and strength, together with a great formability, make them key materials for lightweight structural applications. Relevant examples are the emerging fields of sustainable mobility and wind energy. The efforts were therefore aimed at understanding and modelling the damage mechanisms induced by fatigue, considering the effect of peculiar aspects of fabrics, from the reinforcement architecture and the number of layers to shifting and nesting phenomena. The experimental activities started from the manufacturing and testing of single woven layer specimens with different architectures. The layup allowed the in-situ quantitative monitoring of the crack evolution, focusing on the effect of the weaving pattern. The experimental campaign continued testing multilayer woven composites, analysing the interaction between overlapped layers in a nested configuration. Regarding the modelling activities, a multiscale strategy is developed, with the aim of estimating the elastic properties and the local stresses in the bundles, predicting the first crack initiation in woven composites. An analytical model, based on a shear-lag analysis, is proposed, allowing to relate the damage in terms of cracks to the stiffness degradation. Eventually, an analytical damage-based model for the prediction of crack density evolution is developed.
The PhD thesis aims to deepen the fatigue behaviour of textile composites, with particular focus on woven fabrics. The high specific stiffness and strength, together with a great formability, make them key materials for lightweight structural applications. Relevant examples are the emerging fields of sustainable mobility and wind energy. The efforts were therefore aimed at understanding and modelling the damage mechanisms induced by fatigue, considering the effect of peculiar aspects of fabrics, from the reinforcement architecture and the number of layers to shifting and nesting phenomena. The experimental activities started from the manufacturing and testing of single woven layer specimens with different architectures. The layup allowed the in-situ quantitative monitoring of the crack evolution, focusing on the effect of the weaving pattern. The experimental campaign continued testing multilayer woven composites, analysing the interaction between overlapped layers in a nested configuration. Regarding the modelling activities, a multiscale strategy is developed, with the aim of estimating the elastic properties and the local stresses in the bundles, predicting the first crack initiation in woven composites. An analytical model, based on a shear-lag analysis, is proposed, allowing to relate the damage in terms of cracks to the stiffness degradation. Eventually, an analytical damage-based model for the prediction of crack density evolution is developed.

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30

KALEEL, IBRAHIM. "Computationally-efficient multiscale models for progressive failure and damage analysis of composites." Doctoral thesis, Politecnico di Torino, 2019. http://hdl.handle.net/11583/2729362.

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31

Munden, Daniel Christopher. "Development of a Progressive Failure Model for Notched Woven Composite Laminates." Thesis, Virginia Tech, 2018. http://hdl.handle.net/10919/85058.

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As part of the Composite Technology for Exploration (CTE) project at NASA, woven fabric composites are being investigated for their use in Space Launch System (SLS) hardware. Composites are more difficult to analyze than isotropic materials and require more complex methods for predicting failure. NASA is seeking a method for predicting the damage initiation and propagation of woven fabric composites in order to utilize these materials effectively in SLS hardware. This work focuses on notched woven fabric composites under tensile loading. An analytical model consisting of a macro-level failure criterion and damage propagation was developed and implemented in explicit finite element analysis to simulate woven composite materials. Several failure criteria and propagation models were investigated and compared. A response surface was used to better understand the effects of damage parameters on the failure load of a specimen. The model chosen to have best represented the physical specimen used the Tsai-Wu failure criterion. Additional physical tests are needed to further validate the model.
Master of Science
A composite material consists of two or more different materials that are joined together to form a new material with improved properties. Woven fabric composites weave strips of fibers and a bonding material into a pattern to increase the material’s ability to withstand loads in various directions. NASA is seeking a method to predict the conditions under which woven fabric composites will break. A greater understanding of the capabilities of woven fabric composites will help NASA improve the structures involved in space exploration. This work attempts to build an analytical model that can predict the loads under which a woven fabric composite will break in tension. Several different analytical theories were used to model a woven fabric composite and the results were compared with lab tests. One of the theories, the Tsai-Wu failure criterion, was selected as the best representation of the physical specimen. Further additional physical tests are necessary to further validate the analytical model.

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32

Sutherland, Hazel. "The effects of pattern and frequency of stimulation on transformation and damage in mammalian skeletal muscle." Thesis, University of Liverpool, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.367198.

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33

Horton, Brandon Alexander. "Comprehensive Multi-Scale Progressive Failure Analysis for Damage Arresting Advanced Aerospace Hybrid Structures." Diss., Virginia Tech, 2017. http://hdl.handle.net/10919/93961.

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In recent years, the prevalence and application of composite materials has exploded. Due to the demands of commercial transportation, the aviation industry has taken a leading role in the integration of composite structures. Among the leading concepts to develop lighter, more fuel-efficient commercial transport is the Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS) concept. The highly integrated structure of PRSEUS allows pressurized, non-circular fuselage designs to be implemented, enabling the feasibility of Hybrid Wing Body (HWB) aircraft. In addition to its unique fabrication process, the through-thickness stitching utilized by PRSEUS overcomes the low post-damage strength present in typical composites. Although many proof-of-concept tests have been performed that demonstrate the potential for PRSEUS, efficient computational tools must be developed before the concept can be commercially certified and implemented. In an attempt to address this need, a comprehensive modeling approach is developed that investigates PRSEUS at multiple scales. The majority of available experiments for comparison have been conducted at the coupon level. Therefore, a computational methodology is progressively developed based on physically realistic concepts without the use of tuning parameters. A thorough verification study is performed to identify the most effective approach to model PRSEUS, including the effect of element type, boundary conditions, bonding properties, and model fidelity. Using the results of this baseline study, a high fidelity stringer model is created at the component scale and validated against the existing experiments. Finally, the validated model is extended to larger scales to compare PRSEUS to the current state-of-the-art. Throughout the current work, the developed methodology is demonstrated to make accurate predictions that are well beyond the capability of existing predictive models. While using commercially available predictive tools, the methodology developed herein can accurately predict local behavior up to and beyond failure for stitched structures such as PRSEUS for the first time. Additionally, by extending the methodology to a large scale fuselage section drop scenario, the dynamic behavior of PRSEUS was investigated for the first time. With the predictive capabilities and unique insight provided, the work herein may serve to benefit future iteration of PRSEUS as well as certification by analysis efforts for future airframe development.
PHD

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34

Von, Forell Gregory Allen. "Computational and Experimental Study of Degeneration, Damage and Failure in Biological Soft Tissues." BYU ScholarsArchive, 2013. https://scholarsarchive.byu.edu/etd/3951.

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The purpose of this work was to analyze the biomechanics of degeneration, damage, and failure in biological soft tissues both experimentally and computationally to provide insight into tendon or ligament tearing, tendo-achilles lengthening and lumbar spine dysfunction. For soft tissue tearing, experimental studies for calculating fracture toughness were performed and determined that tendons and ligaments are able to completely resist tear propagation. For tendo-achilles lengthening, a damage model was developed to mimic the behavior of the lengthening that occurs as a result of the percutaneous triple hemisection technique. The model provided insight for predicting the amount of lengthening that occurs during the procedure. For lumbar spine dysfunction, a finite element model was validated against experimental testing and simulated using boundary conditions representing physiological loading. The model was able to predict how biomechanical changes can lead to pain and how the prevalence of Schmorl's nodes can be predicted. For each of the situations, the best verification and validation methods were selected and are presented throughout the research to demonstrate the predictive capabilities and limitations of the work. Results of these studies are presented along with how those results influence the clinical endeavors associated with the degeneration, damage and failure of soft tissues.

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35

Martin, Anthony Russell. "Impact damage detection in filament wound tubes using embedded optical fibre sensors." Thesis, Brunel University, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.266542.

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36

Wong, C. W. "Numerical modelling of masonry panels subject to loading from gas explosions." Thesis, Middlesex University, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.337850.

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37

Chowdhury, Khairul Alam. "Damage initiation, progression and failure of polymer matrix composites due to manufacturing induced defects." Texas A&M University, 2003. http://hdl.handle.net/1969.1/5787.

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In polymer matrix composites (PMCs) manufacturing processes can induce de- fects, e.g., voids, fiber misalignment, irregular fiber distribution in the cross-section and broken fibers. The effects of such defects can be beneficial or deleterious de- pending on whether they cause failure suppression or enhancement by localized de- formation processes e.g., crazing, shear yielding and fiber-matrix debonding. In this study, a computational approach is formulated and implemented to develop solu- tions for general boundary-value problems for PMC microstructures that accounts for micromechanics-based constitutive relations including fine scale mechanisms of material failure. The defects considered are voids, and the microstructure is explic- itly represented by a distribution of fibers and voids embedded in a polymer matrix. Fiber is modeled as a linearly elastic material while the polymer matrix is mod- eled as an elastic-viscoplastic material. Two distinct models for the matrix behavior are implemented: (i) DruckerÃ¢ÂÂPrager type Bodner model that accounts for rate and pressure-sensitivity, and (ii) improved macromolecular constitutive model that also accounts for temperature dependence, small-strain softening and large-strain harden- ing. Damage is simulated by the Gearing-Anand craze model as a reference model and by a new micromechanical craze model, developed to account for craze initiation, growth and breakdown. Critical dilatational energy density criterion is utilized to predict fiber-matrix debonding through cavitation induced matrix cracking. An extensive parametric study is conducted in which the roles of void shape, size and distribution relative to fiber in determining damage initiation and evolution are investigated under imposed temperature and strain rate conditions. Results show there are significant effects of voids on microstructural damage as well as on the overall deformational and failure response of composites.

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Bysh, I. N. "The micromechanics of damage and failure in joints bonded with a particle filled adhesive." Thesis, University of Surrey, 1996. http://epubs.surrey.ac.uk/842907/.

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This thesis has identified the failure and damage processes in a particle filled epoxy which is typical of adhesives used industrially. Micromechanical analyses have been carried out to predict the material properties of damaged adhesive, and to investigate the applicability of different failure criteria. The general body of evidence suggests that there is no direct method of predicting the failure load of adhesive joints from the strength and toughness of the adhesive used. Therefore, a favoured approach has been to postulate a failure criterion, and to implement it in the constitutive equation for the adhesive. In contrast, this work has begun from the microstructural modelling of damage, and derived credible failure criteria from this model. The experimental program quantified the adhesive morphology and identified the damage processes that occur in the adhesive prior to failure. Bulk and joint specimens were tested both in-situ in a scanning electron microscope, and on a conventional tensile testing machine. The tests showed that the mechanisms for damage and failure in both joint and bulk form are particle debonding followed by cracking in the matrix. The concept of a representative unit cell of material was used to determine the effects of particle cracking and debonding. In a regular' array of cracked particles, the stiffness remained relatively unchanged in the plane of the cracks, but perpendicular to it, a significant reduction was found. Modelling debonded particles is more complex, because partial contact must be considered in addition to the fully bonded and fully debonded conditions. The unit cell was used to define the elasticity matrix for adhesive containing debonded particles as a function of strain state. The unit cell concept was extended further by including material that obeyed a modified (i.e. hydrostatically sensitive) Von Mises yield criterion. Particle debonding was found to contribute significantly to the hydrostatic sensitivity and to the softening of the adhesive. The unit cell concept was used to implement a strain at a distance failure criteria, using both elastic and plastic material properties. New types of failure criteria also based on the unit cell have been proposed. The criteria relate the strain state in an adhesive joint to the likelihood of shear banding or tensile plastic flow. The regions in a joint that experience one or the other of the mechanisms were identified. Hence the nature and extent of the adhesive failure in joints with varying joint geometry and loading may be predicted.

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39

陳幸福 and Xingfu Chen. "A ductile damage model based on endochronic theory and its applicationto ductile failure analysis." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1993. http://hub.hku.hk/bib/B31233004.

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40

Wong, M. T. "Three-dimensional finite element analysis of creep continuum damage growth and failure in weldments." Thesis, University of Manchester, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.542702.

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This thesis describes the development of computational analysis techniques for weldments which contain three different material regions (Parent Material, Weld Material and Heat Affected Zone) and which exhibit different creep behaviour. The different strain rate behaviour of the three regions and the growth of creep continuum damage lead to local stress redistribution and to complex states of stress which can greatly enhance the accumulation of creep damage. A review of literature is presented which covers weldment design methodology and the associatedm etallurgy. The shortcomingso f design and assessmenct odes for pipe vessels, and the need for approximate methods of lifetime estimation, are revealed, indicating the need for further research in these areas. The development of the Continuum Damage Mechanics (CDM) method is presented, which is a physically based technique for the analysis of the creep behaviour of materials and engineering structures. Previous works are also reviewed showing that the CDM method can be used to accurately describe the behaviour of weldments providing the constitutive equations are available which describe the deformation and rupture of materials. Since the research is concerned with the analysis of the creep rupture of welded pressure vessels and pipes using the CDM method, previous research is reviewed regarding the creep behaviour of weldments and 'the determination of constitutive equations for different weldment material phases. A review is also presented of different solution techniques for solving systems of equations and for minimising the bandwidth and profile of matrix. The modification is described of the two-dimensional (2D) solver, Damage XX- 2D, to extend its capability to three-dimensional (3D), together with the techniques required to satisfy plastic incompressibility using a special brick arrangement of tetrahedral elements. The Three-dimensional CDM Finite Element Solver is known as Damage XX-3D. The 3D finite element theory and the co-ordinate transformation techniques are outlined for the solution of axi-symmetricengineering problems. The technique for the removal of failed finite elements is described for both of the two-dimensional and the three-dimensional analyses. The restart facility and the associated data output strategy are developed to help to minimise the loss of result data files, in the case of an accidental power cut to computers. These methods also allow a complete analysis to be divided into individual smaller analyses. Two numerical solution methods, with different storage schemes, for sets of linear algebraic equations have been developed and validated against results obtained independently using a commercial Finite Element package Abaqus (version 5.6-1). A damage averaging technique is developed to reduce the number of iterations required for the solution of three-dimensional problems which have large number of degrees of freedom; and also to preserve the symmetry of creep CDM solutions for axi-symmetric two-dimensional analyses. The creep CDM solutions obtained using Damage XX-3D are compared with the solutions obtained using Damage XX-2D Axi-symmetric analysis, and good agreement has been obtained for lifetimes and failure mechanisms. Applications of Damage XX-3D, are presented for the analysis of the high temperaturec reep behaviour of a CrossweldedT estpiece,t he Cylinder-SphereP ipe Intersection (Flank Section) subjected to an internal pressure, and Butt-welded ferritic steel pipe subjected to a combined internal pressure and a global bending moment. Finally, a three-dimensional Finite Element CDM Solver has been developed which is computationally fast and efficient, and which yields predictions which have been validated against independent solutions. IV

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41

Trivellato, Edoardo. "Softening damage contributions to the failure zone around deep tunnels in quasi-brittle rocks." Thesis, Paris Est, 2018. http://www.theses.fr/2018PESC1170.

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L'Agence nationale pour la gestion des déchets radioactifs (Andra) gère un laboratoire souterrain de recherche au Centre de la Meuse/Haute-Marne (CMHM), pour étudier la faisabilité d’un stockage géologique profonde dans l’argilite du Callovo-Oxfordien. Les galeries suivent les deux contraintes horizontales principales majeure et mineure. Tandis que certaines galeries possèdent un état de contraintes presque isotrope dans leur section, les autres montrent une anisotropie plus importante.Ces travaux étudient les phénomènes de rupture et fracturation à court terme, induits par l’excavation autour des ouvrages. L’endommagement fragile est considéré comme mécanisme fondamental de rupture. En effet, on retrouve dans la littérature scientifique une estimation de cette zone à partir d'un post-traitement du champ des contraintes en élasticité ou basée sur un calcul élasto-plastique. Si la première méthode ne considère pas la redistribution des contraintes due aux phénomènes dissipatifs, les approches élasto-plastiques semblent parfois insuffisantes pour expliquer la géométrie de cette zone dans certains cas des excavations en roches quasi-fragiles (Pouya et al 2016). Deux phénomènes sont étudiés, avec des simulations numériques aux Eléments Finis 2d : le développement d’une rupture diffusé, autour de la section des galeries, et l’apparition des fractures le long de l’excavation. Alors qu’une modélisation en déformations planes simule le premier cas, le deuxième est étudié en axisymétrie.Pour la première approche, deux décharges isotropes, en élasto-plasticité ou élasto-endommagement radoucissant, démontrent une instabilité globale liée au premier modèle, alors que des ruptures localisées se produisent avec le deuxième. Ceci, enrichi pour considérer les anisotropies d’élasticité, de résistance et d’endommagement du matériau, permet une bonne estimation de la zone de rupture à court terme, avec des valeurs des convergences conformes aux données, pour une galerie testée comme cas d’étude. Ensuite, la transition de la rupture du matériau de fragile à ductile, avec le confinement, est aussi prise en compte, avec les différentes conditions aux limites d’une deuxième galerie. Les zones endommagées à court terme estiment correctement la forme et l'extension de la rupture et les valeurs des convergences instantanées sont comparables aux mesures (Trivellato et al 2018).Le deuxième axe de recherche concerne la prédiction de la longueur des fractures, simplifiées comme des discontinuités finies et parallèles, ainsi que leur espacement. Numériquement, ces sont assimilées aux éléments joints (Goodman 1976). Le modèle de la fracture cohésive (Pouya et Bemani 2015) a été choisi pour ces éléments. Avec une seule discontinuité, on observe une initiation instable de la fracture, possiblement suivi par une évolution stable, d’une longueur réduite. Plusieurs séquences des joints ont été employés pour calculer la périodicité des fractures. Ces travaux ont permis aussi l’étude de différentes méthodes numériques qui simulent l’avancement d’un front d’excavation. Selon un choix précis des paramètres, on calcule des longueurs de fracturation comparables aux mesures, ainsi que la possibilité d’obtenir l’activation d’une seule fracture parmi plusieurs (Trivellato et al 2018).Ces travaux constituent une nouvelle approche de prédiction des effets du creusement sur l’argilite du Callovo-Oxfordien. Ils étudient une rupture à court terme due au comportement fragile sous faible confinement. Le mécanisme de dissipation en endommagement est intégré par les effets à la fois de l’anisotropie intrinsèque du matériau et de sa transition fragile-ductile. Les résultats obtenus par les deux axes de recherche favorisent l’utilisation de ce modèle comme complément aux études des excavations. En perspective, on peut envisager d’intégrer ces modèles avec les effets de la plasticité/fluage du matériau ainsi qu’avec la poro-élastique en comptant les effets hydrauliques
The French National Radioactive Waste Management Agency (Andra) manages an Underground Research Laboratory (URL) at the Meuse / Haute-Marne Center to study the feasibility of a deep geological repository in the Callovo-Oxfordian claystone (COx). The galleries follow the major and minor principal stress directions. Some galleries show a quasi-isotropic stress state in their cross-section, while others show a greater anisotropy.These works study the short-term failure and fracturing phenomena induced by the underground structures’ excavation. Brittle softening damage is considered as a fundamental failure mechanism. The major part of scientific literature reports an estimation of this area from an elastic post-treatment or based on elastic-plastic analyses. If the first method does not consider the stresses’ redistribution due to dissipative phenomena, elastic-plastic approaches sometimes seem insufficient to explain the geometry of these zones in some cases of deep excavations in quasi-brittle rocks (Pouya et al 2016). Two phenomena are studied, through numerical simulations by 2d Finite Elements: the development of a diffused failures, around the galleries’ cross-section, and the fractures occurrence at their perimeter, along the excavation. While a plane strain analysis is suitable for the first problem, the second one is studied in axial symmetry.For the first part of the research, two isotropic unloading processes, in softening elastic-plasticity or elastic-damage, reproduce a global instability related to the first modelling, whereas localized failure occurs with the second one. The latter model, upgraded to consider intrinsic anisotropies in terms of elasticity, resistance and damage dissipative law, allows a consistent estimation of the short-term failure zone, with values of convergences in accordance to the data, for a gallery considered as a case study. Then, the transition of material’s failure from a brittle to ductile behaviour, with the confinement, is also simulated, according to different boundary conditions of a second gallery. The short-term damaged zones well reproduce the shape and extension of failure systems and the values of the instantaneous convergences are comparable to in-situ measurements (Trivellato et al 2018).The second research axis concerns the elongation and mutual spacing of fractures, simplified as a system of finite and parallel discontinuities. Numerically, they are assimilated to joint elements (Goodman 1976). The cohesive fracture model, based on damage, (Pouya and Bemani 2015) was chosen for these elements. In presence of a single potential fracture, an unstable initiation is observed, possibly followed by a stable evolution, of reduced length. Then, models with a sequences of several joints were used to analyse the fracture periodicity. This work also allowed the study of different numerical techniques simulating the advancement of an excavation front. According to a precise choice of parameters, fractures’ lengths are comparable to the geological and geophysical surveys. Similarly, the activation of one potential fracture among different discontinuities was calculated, showing a periodic occurrence (Trivellato et al 2018).This dissertation constitutes a new approach to reproduce the immediate effects of deep excavations in the Callovo-Oxfordian claystone. They study a short-term failure due to the material’s brittleness, under low confinement. Damage is adopted as the only dissipation mechanism and is integrated by the effects of material’s intrinsic anisotropy as well as its brittle-ductile post-peak transition. Results obtained by every research axis appear favourable to employ these models as complements to excavation studies. In perspective, the integration of plasticity / creep effects, as well as a poro-elastic framework accounting for hydraulic effects, may be considered

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42

Pelà, Luca. "Continuum damage model for nonlinear analysis of masonry structures." Doctoral thesis, Universitat Politècnica de Catalunya, 2009. http://hdl.handle.net/10803/30327.

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The present work focuses on the formulation of a Continuum Damage Mechanics model for nonlinear analysis of masonry structural elements. The material is studied at the macro-level, i.e. it is modelled as a homogeneous orthotropic continuum. The orthotropic behaviour is simulated by means of an original methodology, which is based on nonlinear damage constitutive laws and on the concept of mapped tensors from the anisotropic real space to the isotropic fictitious one. It is based on establishing a one-to-one mapping relationship between the behaviour of an anisotropic real material and that of an isotropic fictitious one. Therefore, the problem is solved in the isotropic fictitious space and the results are transported to the real field. The application of this idea to strain-based Continuum Damage Models is rather innovative. The proposed theory is a generalization of classical theories and allows us to use the models and algorithms developed for isotropic materials. A first version of the model makes use of an isotropic scalar damage model. The adoption of such a simple constitutive model in the fictitious space, together with an appropriate definition of the mathematical transformation between the two spaces, provides a damage model for orthotropic materials able to reproduce the overall nonlinear behaviour, including stiffness degradation and strain-hardening/softening response. The relationship between the two spaces is expressed in terms of a transformation tensor which contains all the information concerning the real orthotropy of the material. A major advantage of this working strategy lies in the possibility of adjusting an arbitrary isotropic criterion to the particular behaviour of the orthotropic material. Moreover, orthotropic elastic and inelastic behaviours can be modelled in such a way that totally different mechanical responses can be predicted along the material axes. The aforementioned approach is then refined in order to account for different behaviours of masonry in tension and compression. The aim of studying a real material via an equivalent fictitious solid is achieved by means of the appropriate definitions of two transformation tensors related to tensile or compressive states, respectively. These important assumptions permit to consider two individual damage criteria, according to different failure mechanisms, i.e. cracking and crushing. The constitutive model adopted in the fictitious space makes use of two scalar variables, which monitor the local damage under tension and compression, respectively. Such a model, which is based on a stress tensor split into tensile and compressive contributions that allows the model to contemplate orthotropic induced damage, permits also to account for masonry unilateral effects. The orthotropic nature of the Tension-Compression Damage Model adopted in the fictitious space is demonstrated. This feature, both with the assumption of two distinct damage criteria for tension and compression, does not permit to term the fictitious space as “isotropic”. Therefore, the proposed formulation turns the original concept of “mapping the real space into an isotropic fictitious one” into the innovative and more general one of “mapping the real space into a favourable (or convenient) fictitious one”. Validation of the model is carried out by means of comparisons with experimental results on different types of orthotropic masonry. The model is fully formulated for the 2-dimensional case. However, it can be easily extended to the 3-dimensional case. It provides high algorithmic efficiency, a feature of primary importance when analyses of even large scale masonry structures are carried out. To account for this requisite it adopts a strain-driven formalism consistent with standard displacement-based finite element codes. The implementation in finite element programs is straightforward. Finally, a localized damage model for orthotropic materials is formulated. This is achieved by means of the implementation of a crack tracking algorithm, which forces the crack to develop along a single row of finite elements. Compared with the smeared cracking approach, such an approach shows a better capacity to predict realistic collapsing mechanisms. The resulting damage in the ultimate condition appears localized in individual cracks. Moreover, the results do not suffer from spurious mesh-size or mesh-bias dependence. The numerical tool is finally validated via a finite element analysis of an in-plane loaded masonry shear wall.

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43

Salih, Anmar Mahdi. "Characterization of In-Vivo Damage in Implantable Cardiac Devices and the Lead Residual Properties." Wright State University / OhioLINK, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=wright1557851495921852.

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44

Wang, Ying. "Damage mechanisms associated with kink-band formation in unidirectional fibre composites." Thesis, University of Manchester, 2016. https://www.research.manchester.ac.uk/portal/en/theses/damage-mechanisms-associated-with-kinkband-formation-in-unidirectional-fibre-composites(0ba9a3a1-78e7-42f6-b0e7-f56dddfce2ca).html.

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The compressive strength of unidirectional (UD) carbon fibre reinforced plastics (CFRPs) is often only 60-70% of their tensile strength owing to premature failure associated with kink-band formation. The sudden and complex nature of kink-band formation has been hindering the progress in experimental studies on the evolution of damage in compressive failure. A better understanding of the damage mechanisms associated with kink-band formation can help to design more reliable composite structures. Therefore, the principal aim of this project is to identify, in three dimensions (3D), the key damage mechanisms underlying the initiation and propagation of kink bands in UD carbon fibre/epoxy composite. A new manufacturing method is developed to fabricate high-quality UD T700/epoxy cylindrical rods for axial compression tests and high-resolution imaging of kink bands by post mortem and in situ X-ray computed tomography (CT). The morphology of kink bands is visualised in 3D by segmenting fibre breaks at kink-band boundaries and representative longitudinal splits. The geometrical parameters of each fully developed kink band are consistent through the specimen. Radiographs obtained from ultra-fast synchrotron imaging show that a kink band initiates and propagates across the specimen in less than 1.2 ms. A scenario of kink-band failure is proposed: fibre buckling and longitudinal splitting occur prior to fibre breakage, which forms kink-band boundaries and eventually the morphology of multiple kink bands develops suddenly. 3D tomographs of the fast and unstable kink-band formation could not be captured in the axial compression experiments. Therefore, a testing method of loading notched UD carbon fibre (T800, T700 and T300)/epoxy beams using a four-point bending (FPB) fixture is developed to enable monitoring of more stable initiation and propagation of kink bands by in situ X-ray CT. Kink-band formation is significantly slowed in the FPB tests. Fibre micro-buckling accompanied by splitting, could initiate the formation of kink bands. In the T700/epoxy system, the early initiation stage of fibre micro-buckling without fracture is captured, and the critical radius of curvature of unbroken fibres prior to fracture is ～130micro metre. Unloading causes significant recovery of fibre curvature (radius of curvature ～280 micro metre) and a reduction of 10-20º in fibre rotation angle within the kink band. The results show that in situ 3D characterisation of kink bands is essential as fibre buckling is a 3D phenomenon, resulting in development of both in-plane and out-of-plane kink bands. Understanding of kink-band formation in 3D will help to establish strategies to improve the compressive strength of CFRP composites by depressing kink-band formation; in this respect lateral constraint conferred by strong interfaces is a key aspect.

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45

Bhana, Vishal Bhooshan. "Online damage detection on shafts using torsional and undersampling measurement techniques." Diss., University of Pretoria, 2013. http://hdl.handle.net/2263/25437.

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The presence of cracks in rotors is one of the most dangerous defects of rotating machinery. This can lead to catastrophic failure of the shaft and long out-of-service periods. The occurrence of a crack in a rotating shaft introduces changes in flexibilities which alters the dynamics during operation. This research deals with detecting damage in rotors by means of constantly monitoring the variation in the rotor’s dynamics during normal operating conditions. This project entails a computer finite element section as well as an experimental investigation. The flexibility in the region of the crack is different from an uncracked section. A finite element model of a shaft is built and investigated. The damaged model is the same except that the nodes in the location of the crack are not equivalenced in order to represent the crack. A simple constant cross-sectional shaft with semi-circular transverse surface cracks varying in size have been modelled on the Patran finite element software and a normal modes analysis was done using the Nastran solver. The results revealed a change in the natural frequencies due to the variation in the size of the crack. The experimental investigation involved creating sample shafts with damage positioned in them that would closely resemble what one may find in actual real-life situations and the dynamics during rotation with various torsional loadings are investigated and monitored using three methods. A fibre-optical sensor, Digital image correlation system and telemetry strain gauges were used. Undersampling techniques were used for the DIC system. Results showed that the fibre-optic sensor is by far the most favourable as it is able to detect damage under constant operation. The finite element model was updated by re-modelling the geometry, damage and material properties. The solution of the analysis matched the experimental results closely and model verification was achieved.
Dissertation (MEng)--University of Pretoria, 2013.
Mechanical and Aeronautical Engineering
unrestricted

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46

Smart, Kevin Arthur. "The mechanism by which hyperammonaemia may cause hepatic encephalopathy." Thesis, King's College London (University of London), 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.267312.

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47

Goyal, Vinay K. "Analytical Modeling of the Mechanics of Nucleation and Growth of Cracks." Diss., Virginia Tech, 2002. http://hdl.handle.net/10919/30006.

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With the traditional fracture mechanics approaches, an initial crack and self-similar progression of cracks are assumed. In this treatise, theoretical and numerical tools are developed to mathematically describe non-self-similar progression of cracks without specifying an initial crack. A cohesive-decohesive zone model, similar to the cohesive zone model known in fracture mechanics as Dugdale-Barenblatt model, is adopted to represent the degradation of the material ahead of the crack tip. This model unifies strength-based crack initiation and fracture based crack progression. The cohesive-decohesive zone model is implemented with an interfacial surface material that consists of an upper and lower surface connected by a continuous distribution of normal and tangential nonlinear elastic springs that act to resist either Mode I opening, Mode II sliding, Mode III sliding, or mixed mode. The initiation of fracture is determined by the interfacial strength and the progression of fracture is determined by the critical energy release rate. The material between two adjacent laminae of a laminated composite structure or the material between the adherend and the adhesive is idealized with an interfacial surface material to predict interfacial fracture. The interfacial surface material is positioned within the bulk material to predict discrete cohesive cracks. The proper work-conjugacy relations between the stress and deformation measures are identified for the interfacial surface theory. In the principle of virtual work, the interfacial cohesive-decohesive tractions are conjugate to the displacement jumps across the upper and lower surfaces. A finite deformation kinematics theory is developed for the description of the upper and lower surface such that the deformation measures are invariant with respect to superposed rigid body translation and rotation. Various mechanical softening constitutive laws thermodynamically consistent with damage mechanics are postulated that relate the interfacial tractions to the displacement jump. An exponential function is used for the constitutive law such that it satisfies a multi-axial stress criterion for the onset of delamination, and satisfies a mixed mode fracture criterion for the progression of delamination. A damage parameter is included to prevent the restoration of the previous cohesive state between the interfacial surfaces. In addition, interfacial constitutive laws are developed to describe the contact-friction behavior. Interface elements applicable to two dimensional and three dimensional analyses are formulated for the analyses of contact, friction, and delamination problems. The consistent form of the interface element internal force vector and the tangent stiffness matrix are considered in the formulation. We investigate computational issues related to interfacial interpenetration, mesh sensitivity, the number of integrations points and the integration scheme, mathematical form of the softening constitutive law, and the convergence characteristics of the nonlinear solution procedure when cohesive-decohesive constitutive laws are used. To demonstrate the predictive capability of the interface finite element formulation, steadystate crack growth is simulated for quasi-static loading of various fracture test configurations loaded under Mode I, Mode II, Mode III, and mixed-mode loading. The finite element results are in agreement with the analytical results available in the literature and those developed in this work. A progressive failure methodology is developed and demonstrated to simulate the initiation and material degradation of a laminated panel due to intralaminar and interlaminar failures. Initiation of intralaminar failure can be by a matrix-cracking mode, a fiber-matrix shear mode, and a fiber failure mode. Subsequent material degradation is modeled using damage parameters for each mode to selectively reduce lamina material properties. The interlaminar failure mechanism such as delamination is simulated by positioning interface elements between adjacent sublaminates. The methodology is validated with respect to experimental data available in the literature on the response and failure of quasi-isotropic panels with centrally located circular cutouts. Very good agreement between the progressive failure analysis and the experiments is achieved if the failure analyses includes the interaction of intralaminar and interlaminar failures in the postbuckling response of the panels. In addition, ideas concerning the implementation of a fatigue model incorporated with a cohesive zone model are discussed.
Ph. D.

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48

Abessalam, Qutaiba. "Investigation into the modes of damage and failure in natural fibre reinforced epoxy composite materials." Thesis, University of East London, 2011. http://roar.uel.ac.uk/2632/.

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The aim of the research is to develop new high performance composite materials that would potentially compete with existing man made materials and to investigate the physical, mechanical and thermal properties of crushed (powdered) olive stones (pits) reinforced epoxy composites. This research focuses on development of a new range of sustainable reinforced polymer composite materials using powdered olive pits as a novel filler material to be used with synthetic resins. A full review has been made of the previous work on different types of natural fibres and fillers used as reinforcement for synthetic polymers. This project attempts to clarify the advantages and limitations resulted from using these fibres/ fillers and endeavours to provide explanation on the mechanical behaviour of these materials. Prior to investigating the mechanical properties of the powdered olive pits-epoxy composites the density and the mechanical properties of the olive pits were fully characterised. The influence of the untreated and treated powder loading (weight fraction) on the void content and the mechanical properties of the composites was examined using different tests, including flexural, tensile, microhardness and impact testing. The composites showed significant improvements in mechanical properties including flexural strength (139%) and flexural modulus (149%), tensile strength (121%) and tensile modulus (46%), microhardness (170%), and impact strength (167%) following treatment of the olive pits powder with 2% Al 100 coupling agent. Composites consisting of epoxy resin reinforced with untreated powder exhibited weaker powder to matrix interfacial bonding compared to those with treated powder composites. The improvements in properties have been attributed to the treatment of the powder with coupling agent, which has resulted in enhanced powdermatrix interaction. Hence it was possible to develop an experimental model of the behaviour of these materials subjected to the above mentioned testing conditions. Furthermore different thermal analysis techniques including dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) have been used to investigate the influence of the coupling agent on the olive pits powder composites properties by carefully analysing the changes in the thermal properties of the composites; glass transition temperature increased by 38%, tan delta peak decreased by 50%, and room temperature storage modulus improved by 17% and loss modulus peak decreased by 60%. The corroborated mechanical and thermal analysis results support the formation of a strong and efficient interfacial bond between the filler and epoxy matrix. The influence of powder content and interfacial bond strength on the damage and failure mechanisms operating in the different samples subjected to the destructive testing regime have been examined using optical and scanning electron microscopy (SEM).

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49

Fuller, L. Bryant. "Damage and compressive failure of unbalanced sandwich composite panels subject of a low-velocity impact." Monterey, California. Naval Postgraduate School, 1994. http://hdl.handle.net/10945/30855.

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50

Kim, Jinkoo. "Micromechanical model for damage and failure of brittle materials : application to polycrystalline ice and concrete." Thesis, Massachusetts Institute of Technology, 1995. http://hdl.handle.net/1721.1/11389.

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Дисертації з теми "Failure and Damage"

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